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Tuesday, March 1, 2016

There has been a lot of talk about this winter being the wettest winter on record, especially because we are supposed to be having an El Niño weather pattern. In this report I am going to talk about how all of these records are set using selective data ranges.

This season is reported as the wettest winter on record for December 1st - February 28th. You might notice that range does not include September, October, or November. November happens to be typically the wettest month of the year in general.

So, I've put together a bunch of data from my weather station, rain data specifically. My station only goes back to the winter season of 2008-2009, so it's not much history to go on compared to historic stations in the area, but it works for this example.

Here is the historic annual rainfall data, by calendar year:

2009: 37.32 inches

2010: 41.95 inches

2011: 41.14 inches

2012: 61.29 inches

2013: 25.69 inches

2014: 42.87 inches

2015: 32.96 inches

There is a wide variance. 2012, for example, was a combination of a wet spring and wet fall.

Here's another look at seasonal annual rainfall, except instead of starting January 1st we are starting the measurement period at August 1st and ending July 31st the next year. I think this date range best captures our annual rain season. I decided to note the El Niño Southern Oscillation status as well, to give you an idea on how it correlates to seasonal precipitation.

El Niño typically results in warmer and drier than normal. La Niña typically is colder and wetter than normal (more snow). ENSO neutral is typically average on everything.

2009-2010 Strong El Niño: 35.80 inches

2010-2011 Strong La Niña: 56.41 inches

2011-2012 Minor La Niña: 45.73 inches

2012-2013 ENSO neutral: 44.09 inches

2013-2014 ENSO neutral: 32.57 inches

2014-2015 ENSO neutral: 33.23 inches

2015-2016 Strong El Niño: 31.08 inches (This season isn't over so measurement is to date)

The numbers come out completely different; of course, it's all about what combination and what configuration you choose to measure the data. You can see how anyone can make a record of any given period of time if they exclude the other portions that were otherwise included previously in the average or total. So when records are talked about, it's important to find out over what period of measurement that record was for.

Overall, we've had a fairly wet winter, not exactly typical of an El Niño year. However the warmer than normal temperatures have been present, keeping lowland snow events unheard of which is typical of El Niño. We were actually drier than normal for September, October, and November, which balances us out for an almost near normal rainy season for the past 6 years at my station.

Normal annual precipitation for Seattle at Sea-Tac is 37.49 inches. The average at Sea-Tac is recorded by the calendar year, starting January 1st, ending December 31st. There is, of course, variance year over year, and in Shoreline we can sometimes get more, as we did 2010-2011 due to frequent convergence zone events. 2013-2015 we were affected by the Blob of warm water in the Northeastern Pacific keeping us much drier and warmer than normal.

For 2015-2016 season, I've graphed out the daily rainfall amounts, and the monthly rainfall totals compared to my station's 6 year average.

Daily Rainfall for 2015-2016 rain season to date.

Monthly rain totals compared to station average.

The first graph starts August 1st, 2015 and ends March 1st, 2016. Between August and November you can see there are only one or two significant rain events. However, starting in November you can see the periods of several days of heavy rain with breaks in between all the way through March. Those periods of heavy rainfall probably represent the majority of our atmospheric river events that occurred this winter.

In the second graph you can see how August is over double its average, all due to that August 29th freak windstorm we had. Sometimes it only takes one single event to blow out an average. Additionally I'd like to point out that the graph shows September and October at nearly half of the average, with November just under the average for rainfall.

December, January and February are all above average, with December and January almost double the average. If you base your rainfall comparison to just December, January and February, then yes, it easily is one of the wettest years for that period of time.

I wouldn't call this a record breaking winter for rainfall, it's only a record breaker if you only look at December, January and February.

Long range outlook: We are still in a strong El Niño pattern. With that we are still seeing the heavy storms moving through, so I expect this pattern to continue through spring. Temperatures will probably be about one, maybe two degrees Fahrenheit above average and rainfall will probably be near or above normal.

A La Niña transition appears likely this summer. That's good news if you don't like the hot weather we had last summer. I don't think we'll see 90°F more than a couple days, and we will probably see a lot of mid-70's, and maybe a day or two with some thunderstorms.

Tuesday Windstorm and Thunder: Our first strong storm system of the week moved through Tuesday afternoon bringing with it winds gusting up to 52mph around the Shoreline and Lake Forest Park area. The strong winds prompted the National Weather Service in Seattle to issue a Wind Advisory in effect from Noon - 9pm. North City Elementary recorded a gust of 37mph, there are also reports that the Edmonds Waterfront had a gust of 52mph. Strong moist westerly flow behind the cold front produced a thunderstorm that moved quickly through the area at around 6:30pm.

Near term forecast (this week): Wednesday should be relatively calm with a few showers around in the morning. Thursday will have more showers before another strong Pacific storm moves in Friday. The storm on Friday is expected to bring with it more wind and more rain. Rain is likely to be steady and continue through the weekend and the early part of next week. Temperatures will generally be the same every day, highs in the 50's and lows in the 40's.